The invention is directed to solar cells, such as multi-junction solar cells made by wafer bonding and layer transfer.
There is considerable interest in the design and fabrication of tandem multi-junction solar cells for high efficiency photovoltaics for space-based and terrestrial applications. Multi-junction solar cells consist of two or more p-n junction subcells with band gaps engineered to enable efficient collection of the broad solar spectrum. The subcell band gaps are controlled such that as the incident solar spectrum passes down through the multi-junction solar cell it passes through subcells of sequentially decreasing band gap energy. Thus, the efficiency losses associated with single-junction cells—inefficient collection of high-energy photons and failure to collect low-energy photons—are minimized.
Multi-junction solar cells are typically fabricated using a monolithic, epitaxial growth process that leads to series connected layers that form subcells of the device. Electrical connection between subcells is performed by a heavily doped tunnel junction formed during the growth of the multi-junction structure. Because of the series-wired nature of such cells, it is important for the photo-current generated by solar radiation in each subcell to be closely matched to all other subcells in the multi-junction solar cell structure.
The monolithic growth process imposes significant limitations on the materials that can be incorporated into multi-junction solar cells. More specifically, MBE and MOCVD manufacturing methods require that any deposited layers be substantially lattice matched to the previous layer or underlying substrate, a restriction that significantly constrains both the substrate choice and the available subcell bandgaps. The resulting constraints have a significant impact on both the photoelectric conversion efficiency and specific power of multi-junction solar cells.